Increased levels of activity and longer half-lives require increased measures to be taken to isolate the waste from inadvertent human intrusion and to minimize the migration of activity back to the biosphere. Increasing depth of disposal with increasing hazard level of the waste is a key parameter used to achieve these goals. However, it is noted that depth is just one of the factors that should be considered for the safety of waste disposal: the properties of the host rock formation, the waste characteristics and engineered features of the facility, regulatory constraints, national policy, etc. are other factors of equal or greater importance.
57 mined repositories. A depth of 30 m is used to differentiate between near surface disposal and disposal at intermediate and greater depths. This depth is widely accepted as the lower level of the ‘normal residential intrusion zone’ (i.e., a depth beyond which human intrusion is limited to drilling and significant excavation activities, such as tunnelling, quarrying and mining) [60]. Deep facilities are generally considered at depths greater than about 300 m (depths generally associated with geological repositories) and intermediate depth facilities in the range from about 30 to 300 m below the surface.
These depths only serve as examples as site specific conditions and safety assessments will dictate the actual facility depth and the need for an engineered barrier system (EBS). The combination of engineered barriers and natural barriers can contain radioactive material until it has decayed to insignificant levels, and provide sufficient isolation and containment to ensure an adequate level of protection for people and the environment.
In the absence of institutional control, a depth of 30 m is considered the minimum necessary to achieve waste isolation. This should therefore be the minimum depth required for waste that might constitute a security risk. However, for waste that would otherwise be eligible for near surface disposal and for short lived radionuclides (where the waste may no longer constitute a hazard after, perhaps, one hundred years) another option is disposal at a shallower depth together with institutional control. Engineered anti-intrusion barriers that are mechanically strong and heavy may also be useful in enhancing isolation.
Higher activity and longer lived waste requires a greater degree of isolation [15, 16].
Specifically, radium, americium and plutonium are of particular concern for disposal because the half-lives of these radionuclides are longer than the period over which many engineered containment features will be effective. Consideration of greater depths and the use of or enhancement of engineered barriers raises the possibility of using intermediate depth and deep geological repositories.
The main features of various disposal approaches are given in Table 16.
TABLE 16. WASTE DISPOSAL METHODS
Disposal method Features Limitations Waste subject to
disposal Landfill sites used for
domestic and industrial wastes
Simple and easy to construct and operate
No institutional control for disposed wastes
Existing facilities can be used
Poor containment and isolation
Exempt waste VLLW
Near surface facilities Simple near surface facilities (trenches)
Excavated trenches covered with a layer of soil
Simple and inexpensive Used historically for short lived LLW
Activity concentration limits should be established
Erosion, intrusion and percolation of rainwater may affect the
performance
Decay to negligible levels during institutional control period (e.g. 100–
300 years) is required Risk of fast migration of radionuclides to biosphere
Waste containing radionuclides with very short half-lives
(VSLW), e.g. those often used for research and medical purposes
Disposal method Features Limitations Waste subject to disposal Engineered near
surface facilities
Multi-barrier approach to enhance the safety of disposal Engineered vault repositories Long experience with operation
Ease of waste emplacement and increased efficiency in the management and closure of the repository
Institutional control (e.g.
100–300 years) is required
Erosion, intrusion and percolation of rainwater may affect the
performance
LLW with short lived radionuclides at higher levels of activity concentration, and also long lived
radionuclides, but only at relatively low levels of activity
concentration.
Near surface borehole or shaft facilities
Alternative or complementary to near surface vaults.
Economical option and also minimizing the probability of human intrusion
Size and quantity of waste packages is limited Institutional control for up to e.g. 300 years is required
DSRS
Intermediate depth facilities
Intermediate depth shafts or boreholes without EBS
Attractive disposal option for small volumes of waste such as radioactive sources The depth is adequate to eliminate the risk of erosion, intrusion and percolation of rainwater
Flexibility in design Possibility to use existing disused cavities (e.g. mines)
Limited or no contact between percolating water is required
Applicable in very low permeability host rocks, with little or no advection of groundwater
Good backfilling and sealing is required Extensive characterization of the site required
Disused sealed radiation sources such as 90Sr, 137Cs, 238Pu and
241Am
Intermediate depth shafts or boreholes with EBSs
Attractive disposal option for small volumes of waste such as radioactive sources
Significant water inflow or the geotechnical characteristics of the geological materials is allowed
Waste containers and packages are important elements in the EBS
Disused high activity sealed sources
Intermediate depth repositories
Massive concrete vaults or silos, with additional EBSs such as clay backfills and buffers
High cost
Extensive characterization of the site required
ILW – waste that will not decay sufficiently within the period of institutional control
Deep facilities
Deep boreholes without EBSs
Containment of radionuclides is provided by the geological barrier
No requirement for supplementary EBSs Lower flow, more stable chemistry and longer potential return paths to the biosphere
High cost
Limited volumes of disposed waste
Disused high activity and long half-life radioactive sources
Deep boreholes with EBSs
Containment of radionuclides is provided by the geological barrier
Use of higher flow
environments encountered in more permeable geological
High cost
Limited volumes of disposed waste
Disused highactivity and long half-life radioactive sources
59
Disposal method Features Limitations Waste subject to
disposal Mined geological
repositories
May comprise caverns or tunnels with varying types of EBSs
Containment of radionuclides is provided by the geological barrier
Suitable for all waste categories
Enhanced confinement
No operational experience for HLW and SFW High capital cost
Assurance of site integrity for above 10 000 years required
Extensive safety and performance analyses required
Suitable geological media required
High level vitrified waste and encapsulated spent fuel
Long lived LILW Disused sources of any activity and half-life